Chemistry and Physics. 101 



liquid cathodes and aluminium anodes, with grooved solid 

 cathodes, and with perforated cathodes. Four parts of Aston's 

 summary especially merit quotation. " (1) The relations between 

 the values of pressure, voltage, current, and the length of the 

 dark space .... satisfy the same form of equations as 

 those previously given for aluminium, the constants varying con- 

 siderably. (2) Roughness of the cathode surface does not appear 

 to affect the discharge, if the dimensions of the irregularities are 

 small compared with the length of the dark space. (3) The 

 length of the dark space is shown, in the cases examined, to be 

 greatest for silver and least for magnesium, the metals following 

 the same order as in the case of the cathode fall. (4) The rate of 

 change of length of the dark space with change of current density 

 at the surface of the cathode seems much the same for all 

 cathodes." — Vide supra, p. 437. h. s. it. 



9. Multiply -charged Atoms. — In the photographs of positive 

 rays previously obtained by Sir J. J. Thomson the mercury line 

 is characterized by the exceptionally small displacement of the 

 head of its parabola. By using gradients as high as 10,000 volts 

 per centimeter Thomson has been able to show that the electro- 

 static displacement for mercury is one-eighth of the normal value 

 for the heads of the parabola corresponding to other elements. 

 The displacement due to the electric field is inversely propor- 

 tional to the kinetic energy of the particle displaced, consequently 

 the atoms which produce the head of the mercury parabola must 

 have eight times the maximum amount of energy possessed by 

 the normal atoms. This could be accounted for if some of the 

 mercury atoms in the discharge-tube had lost eight corpuscles, for 

 then the energy communicated to the atom by the electric field 

 would be eight times the energy imparted to a normally charged 

 atom. Three is the largest multiple of charge heretofore observed 

 by Thomson, for other elements. The photograph reproduced in 

 the paper shows seven parabolas pertaining to mercury. The 

 eighth doubtless exists but is too faint for detection since, in gen- 

 eral, the curves become weaker as the multiple charge increases. 



A careful study of the negatives has led to the conclusion that 

 there are two, and only two, kinds of ionization in the discharge 

 tube. In one of these kinds the mercury atom loses 1 corpuscle, 

 while in the other it loses 8. Moreover, there are no indications 

 of ionization of such a sort as to deprive the mercury atom of 

 7, 6, 5, 4, 3, or 2 corpuscles. Positive charges between the limits 

 8 and 1 arise from the atom first losing 8 corpuscles in the dis- 

 charge tube and subsequently regaining from 1 to 6 corpuscles 

 respectively. Thomson's conception of the processes of ionization 

 is stated very clearly, as follows : " In the first method the ion- 

 izing agents are the rapidly moving corpuscles which constitute 

 the cathode rays, these very small particles penetrate into the 

 atom and come into collision with the corpuscles inside it individ- 

 ually, the collision in favorable cases causing the corpuscles struck 

 to escape from the atom ; this type of ionization results in the 



